Vertical cavity surface emitting lasers are well known to the person skilled in the art. Such lasers have become useful devices for a variety of applications, for instance, in telecommunications, optical property sampling and measurement technology, such as in spectrometers, or in optical position detection devices, such as optical encoders or in optical computer mice.
The VCSEL is a very attractive device for an optical light source suitable for mass production, in that a large number of VCSELs can be simultaneously fabricated on a single wafer by well established process technology. The VCSELs can, additionally, be provided with integrated circuitry associated and operable for driving the VCSEL. Therefore, VCSELs are very attractive candidates for providing low cost laser light sources for a variety of applications, including those mentioned above.
In some of these applications of a laser light source, it may be very important to precisely monitor the output power generated by the VCSEL. This requirement is addressed in US 2005/0286593, in which a monitor photodiode is monolithically integrated beneath a VCSEL, being sandwiched between the VCSEL and the substrate. The VCSEL is comprised of an active layer sandwiched between two distributed Bragg reflectors (DBR) acting as the cavity mirrors for the laser. The DBR mirrors on both sides of the active layers are highly reflective, but still allow a small percentage of radiation of about 0.5% to pass through so as to be able to emit light through the top of the device, and also to allow some light to pass down to the photodiode. The photodiode is integrated on top of the substrate, followed by a tunnel diode, on top of which the VCSEL is formed. The document addresses the problem of reducing the amount of spontaneous emission that reaches the photodiode. As a solution, it is suggested to provide layers in the bottom DBR mirror with a high gallium fraction, the amount thereof being optimized such that these layers become heavily absorbing below the lasing wavelength. Thus, in order to suppress the amount of spontaneous emission that is detected by the photo detector, this teaching exploits the difference in bandwidth between the narrow bandwidth coherent laser light and the broad bandwidth spontaneous emission light, in order to preferentially absorb the latter.
Due to its cavity being formed by plane mirrors generated by DBRs, the vertical cavity surface emitting laser cannot employ so-called “stable resonators” which would require concavely curved mirror surfaces. Instead, the resonator of the VCSEL is just at the edge of stability, wherein stable operation can only be obtained due to the very short length of the cavity and of the active medium. The more the cavity of a laser fulfills the stability criterion the higher is the percentage of coherent emission.
At the lasing threshold the amount of spontaneous emission emitted from a VCSEL may be quite substantial, in the range of 1 percent or even more of the laser total output power at a typical laser power level of about 1 mW, which is a typical operating level for a VCSEL, for instance, in a pointing device such as a computer mouse.
Such high levels of spontaneously emitted incoherent light, have a broader bandwidth than the coherently emitted laser light, and thus is less efficiently reflected by the DBRs, providing a high amount of noise and thus a substantial error source for the proper detection of the output power of the laser.
Additionally, if the laser and photodiode are integrated, either monolithically or in a hybrid manner, further reflecting surfaces are provided which back-reflect radiation from the photodiode into the laser cavity, thereby causing unwanted alteration of the laser properties in an uncontrolled way. Inevitable variation of the substrate thickness, substrate surface roughness, variation of the thickness of the layers of the photodiode, or, as in the case of the arrangement according to US 2005/0286593 A1 of the intermediate tunnel diode, will cause an undefined phase of the light reflected back into the laser cavity, thereby disturbing the light modes amplified within the laser cavity.
Accordingly, it is the object of the present invention to provide a new and improved optoelectronic device overcoming the aforementioned problems of the prior art and allowing precise monitoring the output power of the optoelectronic device without disturbing the efficiency and performance of the generation of laser light output from the vertical cavity surface emitting laser.